The crew module is a separate pressurized structure suspended by links within the forward fuselage structure. This design provides multiple barriers against penetration by meteoroids or orbital debris from most directions except through the crew module aft bulkhead. The aft bulkhead constitutes the forward boundary of the payload bay and, unlike the rest of the crew module, is not shielded by an outer shell. To support missions to the ISS, however, three of the four orbiters will be outfitted with an external airlock that will provide some shielding of the crew compartment aft bulkhead from impacts by meteoroids or orbital debris. The crew module has 11 windows, each made up of primary alumino-silicate and redundant silica pressure panes. Nine windows have additional outer fused-silica “thermal” panes (Smith, 1995).

The majority of orbiter subsystems that do not need to be in the crew compartment are located along the fore-aft axis, as shown in Figure 2–4. Among the components on this axis are tanks of liquid oxygen, liquid hydrogen, and pressurized gases, as well as hydraulic lines running forward to the nose gear well for lowering, braking, and steering the nose wheel. Most of the components located in the mid-fuselage are located below a payload bay liner, which covers the major frames. When the payload bay doors are open on orbit, the sill longerons and cable trays, which run the full length of both sides, still provide some measure of protection for the pressure vessels and other components below them. Payloads carried in the payload bay can also provide shielding for these areas. TPS tiles and aluminum skin stringer panels provide protection from below.

The payload bay doors are built of graphite-epoxy and are opened for payload operations on orbit. Attached to these doors, and exposed to the meteoroid and orbital debris environment, are the radiators of the active thermal control system. These radiators are constructed of aluminum honeycomb material, with internal lines that carry a freon coolant fluid from the heat exchangers.

The orbiter wing structures are generally devoid of any internal systems hardware, except for the main landing gear wells and the hydraulic and electrical lines that run outboard to the 11 hydraulic actuators along the inside of the rear wing spar. The leading edge of each wing is comprised of 22 panels of RCC material.

The orbital maneuvering system (OMS) pods are installed atop the aft fuselage astride the vertical tail. They contain both the OMS and the aft reaction control systems, as well as their components and propellant tanks. The graphite-epoxy skin panels are covered by a combination of TPS tiles and blankets. The exposed area of the pods is small, so their susceptibility to the impacts of meteoroids and orbital debris is relatively low.

The aft fuselage contains the members of the thrust structure for the space shuttle main engines, the myriad lines and valves of the main propulsion system, the auxiliary power units, and components of several other subsystems. The primary structure of the aft fuselage is comprised of aluminum skin and stringer panels and frames. The vertical tail and OMS pods shield the top portion of the aft fuselage. The massive primary thrust structure and propulsion system feedlines



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